Electrochemical glucose test strips, such as those used in the OneTouch® Ultra® whole blood testing kit, which is available from LifeScan, Inc., are designed to measure the concentration of glucose in a blood sample from patients with diabetes. The measurement of glucose is based upon the specific oxidation of glucose by the flavo-enzyme glucose oxidase. During this reaction, the enzyme becomes reduced. The enzyme is re-oxidized by reaction with the mediator ferricyanide, which is itself reduced during the course or the reaction. These reactions are summarized below.D-Glucose+GOx(OX)→Gluconic acid+GOx(RED) GOx(RED)+2 Fe(CN)63−→GOx(OX)+2 Fe(CN)64−
When the reaction set forth above is conducted with an applied potential between two electrodes, an electrical current may be created by the electrochemical re-oxidation of the reduced mediator ion (ferrocyanide) at the electrode surface. Thus, since, in an ideal environment, the amount of ferrocyanide created during the chemical reaction described above is directly proportional to the amount of glucose in the sample positioned between the electrodes, the current generated would be proportional to the glucose content of the sample. A redox mediator, such as ferricyanide is a compound that exchanges electrons between a redox enzyme such as glucose oxidase and an electrode. As the concentration of glucose in the sample increases, the amount of reduced mediator formed also increases, hence, there is a direct relationship between current resulting from the re-oxidation of reduced mediator and glucose concentration. In particular, the transfer of electrons across the electrical interface results in a flow of current (2 moles of electrons for every mole of glucose that is oxidized). The current resulting from the introduction of glucose may, therefore, be referred to as the glucose current.
Because it can be very important to know the concentration of glucose in blood, particularly in people with Diabetes, meters have been developed using the principals set forth above to enable the average person to sample and test their blood to determine the glucose concentration at any given time. The Glucose Current generated is monitored by the meter and converted into a reading of glucose concentration using a preset algorithm that relates current to glucose concentration via a simple mathematical formula. In general, the meters work in conjunction with a disposable strip that includes a sample chamber and at least two electrodes disposed within the sample chamber in addition to the enzyme (e.g. glucose oxidase) and mediator (e.g. ferricyanide). In use, the user pricks their finger or other convenient site to induce bleeding and introduces a blood sample to the sample chamber, thus starting the chemical reaction set forth above.
In electrochemical terms, the function of the meter is two fold. Firstly, it provides a polarizing voltage (approximately 0.4 V in the case of OneTouch® Ultra®) that polarizes the electrical interface and allows current flow at the carbon working electrode surface. Secondly, it measures the current that flows in the external circuit between the anode (working electrode) and the cathode (reference electrode). The meter may, therefore be considered to be a simple electrochemical system that operates in two-electrode mode although, in practice, third and, even fourth electrodes may be used to facilitate the measurement of glucose and/or perform other functions in the meter.
In most situations, the equation set forth above is considered to be a sufficient approximation of the chemical reaction taking place on the test strip and the meter reading a sufficiently accurate representation of the glucose content of the blood sample. However, under certain circumstances and for certain purposes, it may be advantageous to improve the accuracy of the measurement. For example, where a portion of the current measured at the electrode results from the presence of other chemicals or compounds in the sample. Where such additional chemicals or compounds are present, they may be referred to as interfering compounds and the resulting additional current may be referred to as Interfering Currents
Examples of potentially interfering chemicals (i.e. compounds found in physiological fluids such as blood that may generate Interfering Currents in the presence of an electrical field) include ascorbate, urate and acetaminophen (Tylenol™ or Paracetamol). One mechanism for generating Interfering Currents in an electrochemical meter for measuring the concentration of an analyte in a physiological fluid (e.g. a glucose meter) involves the oxidation of one or more interfering compounds by reduction of the enzyme (e.g. glucose oxidase). A further mechanism for generating Interfering Currents in such a meter involves the oxidation of one or more interfering compounds by reduction of the mediator (e.g. ferricyanide). A further mechanism for generating Interfering Currents in such a meter involves the oxidation of one or more interfering compounds at the working electrode. Thus, the total current measured at the working electrode is the superposition of the current generated by oxidation of the analyte and the current generated by oxidation of interfering compounds. Oxidation of interfering compounds may be a result of interaction with the enzyme, the mediator or may occur directly at the working electrode.
In general, potentially interfering compounds can be oxidized at the electrode surface and/or by a redox mediator. This oxidation of the interfering compound in a glucose measurement system causes the measured oxidation current to be dependent on both the glucose and the interfering compound. Therefore, if the concentration of interfering compound oxidizes as efficiently as glucose and/or the interfering compound concentration is significantly high relative to the glucose concentration, it may impact the measured glucose concentration.
The co-oxidization of analyte (e.g. glucose) with interfering compounds is especially problematic when the standard potential (i.e. the potential at which a compound is oxidized) of the interfering compound is similar in magnitude to the standard potential of the redox mediator, resulting in a significant portion of the Interference Current being generated by oxidation of the interfering compounds at the working electrode. Electrical current resulting from the oxidation of interfering compounds at the working electrode may be referred to as direct interference current. It would, therefore, be advantageous to reduce or minimize the effect of the direct interference current on the measurement of analyte concentration. Previous methods of reducing or eliminating direct interference current include designing test strips that prevent the interfering compounds from reaching the working electrode, thus reducing or eliminating the direct interference current attributable to the excluded compounds.
One strategy for reducing the effects of interfering compounds that generate Direct interference current is to place a negatively charged membrane on top of the working electrode. As one example, a sulfonated fluoropolymer such as NAFION™ may be placed over the working electrode to repel all negatively charged chemicals. In general, many interfering compounds, including ascorbate and urate, have a negative charge, and thus, are excluded from being oxidized at the working electrode when the surface of the working electrode is covered by a negatively charged membrane. However, because some interfering compounds, such as acetaminophen, are not negatively charged, and thus, can pass through the negatively charged membrane, the use of a negatively charged membrane will not eliminate the Direct interference current. Another disadvantage of covering the working electrode with a negatively charged membrane is that commonly used redox mediators, such as ferricyanide, are negatively charged and cannot pass through the membrane to exchange electrons with the electrode. A further disadvantage of using a negatively charged membrane over the working electrode is the potential to slow the diffusion of the reduced mediator to the working electrode, thus increasing the test time. A further disadvantage of using a negatively charged membrane over the working electrode is the increased complexity and expense of manufacturing the test strips with a negatively charged membrane.
Another strategy that can be used to decrease the effects of Direct Interfering Currents is to position a size selective membrane on top of the working electrode. As one example, a 100 Dalton size exclusion membrane such as cellulose acetate may be placed over the working electrode to exclude compounds having a molecular weight greater than 100 Daltons. In this embodiment, the redox enzyme such as glucose oxidase is positioned over the size exclusion membrane. Glucose oxidase generates hydrogen peroxide, in the presence of glucose and oxygen, in an amount proportional to the glucose concentration. It should be noted that glucose and most redox mediators have a molecular weight greater than 100 Daltons, and thus, cannot pass through the size selective membrane. Hydrogen peroxide, however, has a molecular weight of 34 Daltons, and thus, can pass through the size selective membrane. In general, most interfering compounds have a molecular weight greater than 100 Daltons, and thus, are excluded from being oxidized at the electrode surface. Since some interfering compounds have smaller molecular weights, and thus, can pass through the size selective membrane, the use of a size selective membrane will not eliminate the Direct interference current. A further disadvantage of using a size selective membrane over the working electrode is the increased complexity and expense of manufacturing the test strips with a size selective membrane.
Another strategy that can be used to decrease the effects of Direct interference current is to use a redox mediator with a low redox potential, for example, a redox potential of between about −300 mV to +100 mV (vs a saturated calomel electrode). This allows the applied potential to the working electrode to be relatively low which, in turn, decreases the rate at which interfering compounds are oxidized by the working electrode. Examples of redox mediators having a relatively low redox potential include osmium bipyridyl complexes, ferrocene derivatives, and quinone derivatives. However, redox mediators having a relatively low potential are often difficult to synthesize, relatively unstable and relatively insoluble.
Another strategy that can be used to decrease the effects of interfering compounds is to use a dummy electrode in conjunction with the working electrode. The current measured at the dummy electrode may then be subtracted from the current measured at the working electrode in order to compensate for the effect of the interfering compounds. If the dummy electrode is bare (i.e. not covered by an enzyme or mediator), then the current measured at the dummy electrode will be proportional to the Direct interference current and subtracting the current measured at the dummy electrode from the current measured at the working electrode will reduce or eliminate the effect of the direct oxidation of interfering compounds at the working electrode. If the dummy electrode is coated with a redox mediator then the current measured at the dummy electrode will be a combination of Direct interference current and interference current resulting from reduction of the redox mediator by an interfering compound. Thus, subtracting the current measured at the dummy electrode coated with a redox mediator from the current measured at the working electrode will reduce or eliminate the effect of the direct oxidation of interfering compounds and the effect of interference resulting from reduction of the redox mediator by an interfering compound at the working electrode. In some instances the dummy electrode may also be coated with an inert protein or deactivated redox enzyme in order to simulate the effect of the redox mediator and enzyme on diffusion. Because it is preferable that test strips have a small sample chamber so that people with diabetes do not have to express a large blood sample, it may not be advantageous to include an extra electrode which incrementally increases the sample chamber volume where the extra electrode is not used to measure the analyte (e.g. glucose). Further, it may be difficult to directly correlate the current measured at the dummy electrode to interference currents at the working electrode. Finally, since the dummy electrode may be coated with a material or materials (e.g. redox mediator) which differ from the materials used to cover the working electrode (e.g. redox mediator and enzyme), test strips which use dummy electrodes as a method of reducing or eliminating the effect of interfering compounds in a multiple working electrode system may increase the cost and complexity of manufacturing the test strip.
Certain test strip designs which utilize multiple working electrodes to measure analyte, such as the system used in the OneTouch® Ultra® glucose measurement system are advantageous because the use of two working electrodes. In such systems, it would, therefore, be advantageous to develop a meter for use with such test strips in the reducing or eliminating the effect of interfering compounds. More particularly, it would be advantageous to develop a meter for use with such strips in reducing or eliminating the effect of interfering compounds without utilizing a dummy electrode, an intermediate membrane or a redox mediator with a low redox potential.